Marine airguns and seismic surveying methods associated with marine airguns are described in U.S. Pat. Nos. 3,249,177; 3,379,273; 3,653,460 and 4,038,630 which are assigned to the present assignee and to which the reader may refer for background information. With continual and increased emphasis on off-shore exploration, as well as the escalating costs of drilling in bodies of water, better yielding and more accurate seismic information with respect to the formations below the body of water are required. An array of airguns towed behind a survey vessel acts as an antenna array to transmit seismic energy with desired characteristics in controlled directions relative to the array. However, there is a limit to the number of airguns which may be towed due to the unwieldy size and towing drag, stresses and reliability and maintenance problems caused by the umbilical cable conventionally required. Each airgun requires two electrical leads connected to its solenoid valve for firing the airgun as well as two leads to the monitor sensor electrical transducer for detecting the instant of firing of the associated airgun plus the high pressure air feed line.
As more airguns are added, the umbilical cable extending from the ship gets larger and larger until the point where it becomes unmanageable. Also, the large diameter umbilical cable creates a great amount of frictional, turbulent and vibrational "drag" from the water as it is being towed behind a ship. Such large effect drag induces considerable stress on the umbilical cable and its components leading to premature failures. Such large drag effect causes the ship itself to consume more fuel than would occur at a more moderate level.
Furthermore, with such a large bundle of air lines and wires in one umbilical cable, it is difficult to locate electrical or pneumatic faults and difficult to test and repair, involving slicing into the cable, separating and repairing the faulty component, reassembling, and then repairing the incision with time-consuming surgical-like complexities and procedures. Moreover, once a given umbilical cable has been assembled, it is difficult to add further airguns to that particular array. There is very little adaptability in an umbilical-type array.